1. Field of the Invention
The invention relates to syringes for dosed injection of a medicine from an exchangeable cartridge of the kind having a piston which is forced into a tubular cartridge to press out a dose of medicine corresponding to the movement of the piston, the syringe having a housing comprising a cartridge holder and a dosing mechanism by which a dose is set and subsequently injected by successively advancing a piston rod to press the piston into the cartridge, said dosing mechanism comprising a threaded spindle and a nut member cooperating with the spindle so that by setting of a dose relative rotation of the spindle and the nut member will move the nut member along the spindle, the position of the nut member on the spindle defining how far the piston rod is advanced during the injection. The spindle may be formed by the piston rod which may be provided with an outer thread.
2. Description of the Related Art
In known syrgines of such type, when the cartridge is empty said piston rod projects into the cartridge in almost the total length thereof. To change the cartridge the piston rod must first be drawn out of the empty cartridge, and thereafter it must be brought back to its initial position in the dose setting part. The last operation is made possible by locks getting unlocked when the empty cartridge is removed from the syringe, where after the piston rod may be pushed or screwed back to its initial position.
The release of said locking may be obtained either by bringing the inner thread of the nut member out of engagement with the spindle or by allowing a free relative rotation of the nut member and the spindle. When the locking is released the nut member, which have during the injections performed been moved to a position on the spindle corresponding to a fully advanced piston rod, may be moved along the spindle back to its position corresponding to a totally retracted piston rod.
By the free rotation of the nut member relative to the spindle the nut may be screwed back to its initial position corresponding to a fully retracted piston rod. However, the nut has to be rotated about 75 turns and this screwing may demand some handiness. This problem may be overcome by using threads with a high pitch and low friction which allows the spindle to rotate by itself when the nut is pressed in the axial direction of said spindle. Another solution is to use a nut member comprising at least two parts which may be pulled apart so that their threads are drawn out of engagement with the thread of the spindle when the nut is going to be returned to its initial position. However, this solution may cause problems when the parts are brought together again to engage the thread of the spindle as it may be difficult to obtain the correct synchronization between the position of the nut member on the spindle and the scale indicating the dose set. Further, the engagement between the threads is dependent on tolerances which may cause wear which makes the settings inaccurate.